One form of printing plates uses silver images as ink-receptive areas. When such plates are made, e.g., by photographic or mechanical processes, undesired images may be formed, e.g., by dust, fingerprints, stains; or images may be present in an original which are not wanted in all copies to be produced, e.g., serial numbers. In order to obtain good printed matter, undesired portions of the image must be deleted after the plate has been made. Preferably such deletion may be performed either before printing or after several copies have been made.
Many proposals have been made for deleting undesired images. For instance; one method comprises precipitating a hydrophilic metal salt, rubber, latex, or gelatin to produce a hydrophilic layer or "cap" over the undesired silver image. Such caps tend to be insufficiently durable and to wear away, especially during long press runs, causing the "deleted" image to reappear after a few hundred copies or impressions.
Other methods include mechanically removing the silver image by erasers, knives, etc. to expose the underlying hydrophilic plate base, or treating the undesired image with an oxidizing solution, e.g., a solution which contains an oxidant such as a bichromate or ferricyamide and a sulfite or thiosulfate, thereby rendering it hydrophilic.
U.S. Pat. No. 4,433,531 (Yamada et al.) discloses an aqueous deletion solution comprising an s-triazine compound. U.S Pat. No. 3,961,928 (DeLorenzo) discloses an aqueous deletion fluid comprising nitric acid and an inert metal oxide thickening agent. A problem with many oxidizing solutions is that they may achieve only partial deletion or typically require skilled application to achieve satisfactory results. Some silver bleaching agents require subsequent application of an additional formulation to "fix" the deletion. Further, many oxidizing solutions may leave oleophilic by-products. These by-products typically tend to accumulate around an area of deletion as halo-like diffusion rings which contain incompletely oxidized silver, or may be rendered oleophilic through absorption of ink components. Such halos are believed to be the result of migration of the solution through the plate base, outwardly from the point of application. As the solution emerges from within the plate base, it tends to raise or "leach-out" oleophilic silver from within the plate which attracts ink residue, etc. Such halos typically spoil copies produced during the start-up phase of printing, i.e., the first few hundred impressions, tending to wear away thereafter.
A commercial product available from the Minnesota Mining and Manufacturing Company, XL Image Deletion Fluid, contains an aqueous solution comprising a triiodide complex (I.sub.3.sup.-), which is formed from dissolved iodine and iodide ion, and fumed silica as a thickening agent. Deletion is accomplished by rubbing the solution on the undesired image areas, e.g., with a cotton swab, thereby oxidizing the silver image. This action produces relatively oleophilic residues which must then be removed, e.g., by wiping with a clean swab or rinsing the affected area with water. While effective deletion may be achieved with such solutions, residence time or duration of application is critical. Insufficient residence time may result in incomplete deletion whereas an excessive residence time may lead to the formation of the halo-like images discussed above.